Known are thin film transistors (hereinafter referred to as TFT) having an active layer of a silicon film formed on a glass or quartz substrate.
Various types of thin film transistors are known, of which, at present, bottom-gate-type ones are being most industrialized for practical use.
In view of their productivity, it is desirable to develop much in future bottom-gate-type TFT to which the production process in part, the designing rule and the production equipment are common.
A bottom-gate-type, thin film transistor comprises a gate electrode, a gate-insulating film and an active layer of a silicon film as formed on a substrate in that order.
As the silicon film constituting the active layer, generally used is an amorphous silicon film. However, in order to obtain TFT of higher quality, it is desirable to use a crystalline silicon film.
To obtain a crystalline silicon film, much used is a technique of crystallizing an amorphous silicon film through exposure to laser radiations.
For the crystallization, also employable is heating, which, however, does not apply to the production of bottom-gate-type TFT.
This is because, in the process for producing bottom-gate-type TFT, the heating shall follow the formation of the gate electrode, by which the gate electrode material will diffuse unfavorably.
However, from the viewpoint of the quality of the crystalline silicon film produced and of the process of producing the film, heating is preferred to laser irradiation.
As the material of the gate electrode, most preferred is aluminum with low resistance.
However, the electrode material of aluminum is problematic in that it will diffuse, as being influenced by the heating of the active layer for crystallization and activation, to often form projections of so-called hillocks and whiskers.
In particular, in the production of bottom-gate-type TFT, the gate electrode is first formed and thereafter the active layer is formed thereon. In this, therefore, the influence of heat on the layers to be formed in different steps is often problematic.